56 PKACTICAL PHOTO-MICKOGKAPHY 



depreciate the resulting image. It is also important to have an 

 object-slide that is as free as possible from dirt, scratches, and 

 impurities or irregularities in the glass ; any such become equally 

 as visible as the object itself. Even if they are not in the exact 

 object-plane, they result in considerable general diffusion of 

 light which only interferes with the optical perfection of the 

 image. To obtain the best results, the thickness of the object- 

 slide should be determined, and the condenser should be 

 specially made and corrected for its particular thickness. 



In setting up the condenser and putting the cedar-wood oil 

 between it and the object-slide, it is important to see that no 

 air-bubbles are included, as these would have an even greater 

 depreciating effect than any foreign substance or defect in the 

 object-slide itself. The same remarks would apply to cover- 

 glasses, which should be perfectly free from defects, and their 

 thickness must be that for which the objective is corrected. In 

 the case where the objective has a collar-correction, the correc- 

 tion for thickness must be made to suit the cover-glass in use. 



The object itself must be mounted in either water or oil, no 

 result being obtainable if it is mounted in air, or in any turbid 

 or opalescent medium. It should be as thin as circumstances 

 will permit. 



It is usual with annular illumination to employ an 

 objective as low in power as possible, the necessary further 

 amplification of the image being effected by the use of higher 

 power oculars. 



For photographic purposes a high-power source of light is 

 essential ; and, where the exposures are to be short, then the only 

 practical light is an electric arc, or a high-power lime-light. The 

 correct centration of the light in relation to the optical system 

 will, of course, be carried out with the same care as when 

 photographing any other microscopic object. 



It may sometimes happen that the amount of light being 

 transmitted by the marginal portion ol the condenser is greater 

 than the object requires, and the result is seen in the image being 

 generally fogged and with a hazy appearance over the whole 

 object. This may be counteracted by closing the sub-stage iris- 

 diaphragm until the illumination is as desired. 



Another method of dark-ground illumination is obtained 

 by using a suitable condenser and by stopping out the central 



